Flexible metallic glass substrate with high resilience, manufacturing method thereof, and electronic device using same

ABSTRACT

Disclosed herein is a flexible substrate, made of metallic glass that is of high resilience suitable for use in electronic devices. The metallic glass is composed of a commercial alloy that can be produced in a continuous process on a mass scale, and may be selected from among Mg-, Ca-, Al-, Ti-, Zr-, Hf-, Fe-, Co-, Ni-, and Cu-based metallic glass. Preferably, its crystallization temperature, which determines the process allowable temperature, is 200° C. or higher. The flexible metallic glass substrate exhibits excellent fatigue properties as well as resilience of 1.5 MJ/m 3  or higher. Its coefficient of thermal expansion is within a small range of 1 to 20 ppm/° C., so that the flexible metallic glass substrate shows a better interfacial property with electronic devices.

CROSS-REFERENCES TO RELATED APPLICATION

The present application claims priority of Korean Patent Application No.10-2014-0107023, filed on Aug. 18, 2014, which is incorporated herein byreference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a flexible substrate, a manufacturingmethod thereof, and an electronic device using the same. Particularly,the present invention relates to flexible metallic glass substrate thatexhibits excellent fatigue properties as well as high resilience andprocess allowable temperature.

2. Description of the Related Art

In recent years, intensive efforts have been directed toward the use offlexible substrates in electronics and electronic devices.Advantageously, the application of flexible substrates makes it possibleto perform processes continuously, as in a roll-to-roll process, insteadof conventional discontinuous processes. Further, with the growth ofdemand for flexible electronics, attention has been focused on flexiblesubstrates and electronic devices using the same.

As typical flexible substrates, polymer materials and iron-based metalmaterials, such as stainless steel (SUS), are currently used.

Polymers are light and exhibit excellent ductility. In addition,polymers have an elastic limit of at least 2%, which is relativelyhigher than that of other materials, and are not limited as to shape orthickness. Thanks to these advantages, polymers are suitable for use asflexible substrates. On the other hand, polymers are low in phasetransition temperature, such as melting point (T_(m)) and glasstransition temperature (T_(g)), limiting the temperatures available forprocesses of fabricating electronic devices to within 100 to 300° C.Further, since polymers are generally vulnerable to oxygen and water, anadditional waterproof layer is required in order to ensure thedurability of electronics.

In contrast, crystalline metal foil, such as SUS, is stable at hightemperatures, extending the range of temperatures available forelectronic device fabrication processes to up to 1,000° C. In addition,its relatively small coefficient of thermal expansion is advantageousfor the formation of good interfaces with electronic devices.Crystalline metal foil is highly durable not only because it does notallow the permeation of water and oxygen thereto, but also because ithas good impact resistance. However, a metal substrate with a thicknessof 15 to 150 μm, used in flexible substrates, has a surface roughness ofhundreds of nanometers or higher due to the manufacturing methodthereof. Crystalline metal plates manufactured by rolling, for example,have rolling marks thereon, and cannot avoid defects causative ofsurface roughness, such as grain boundaries, due to the properties ofthe crystalline materials themselves. For a thick metal film formed on aglass substrate by deposition, surface roughness increases withthickness. Accordingly, it is difficult to achieve fine surfaceroughness on a metal film because surface roughness varies depending onthe deposition method and conditions. Moreover, because of its lowelastic limit of 0.5% or less, crystalline metal foil is apt to undergoplastic deformation even under small bending and thus to have surfacedefects such as wrinkles, which further deteriorate the surfaceroughness. The application of a polymer resin layer to crystalline metalsubstrates has been suggested in order to planarize and insulate themetal substrates. However, the presence of defects on the crystallinesubstrate requires the formation of a relatively thick polymer layer. Inaddition, the metal substrate retains the intrinsic problem whereby themetal substrate readily allows the occurrence of surface defects thereonbecause, whether coated with the polymer layer or not, it undergoesplastic deformation even when only slightly bent due to its low elasticlimit of 0.5% or less. Alternatively, other efforts including polishingand alternative processing have been made with the goal of planarizingthe surfaces of crystalline metal substrates, but in spite of allattempts, the physical limits of crystalline materials have not yetpermitted results of a satisfactory level.

Document of Related Art Patent Document

-   (Patent Document 1) Korean Patent Unexamined Application Publication    No. 2009-0114195-   (Patent Document 2) Korean Patent Unexamined Application Publication    No. 2006-0134934-   (Patent Document 3) Korean Patent Unexamined Application Publication    No. 2004-0097228-   (Patent Document 4) Korean Patent Unexamined Application Publication    No. 2008-0024037-   (Patent Document 5) Korean Patent Unexamined Application Publication    No. 2009-0123164-   (Patent Document 6) Korean Patent Unexamined Application Publication    No. 2008-0065210-   (Patent Document 7) Korean Patent No. 1271864-   (Patent Document 8) Korean Patent Unexamined Application Publication    No. 2013-0026007

SUMMARY OF THE INVENTION

Accordingly, the present invention has been made keeping in mind theabove problems occurring in the prior art, and an object of the presentinvention is to provide a flexible metallic glass substrate with highresilience, provided with the advantages of both conventional polymermaterials and crystalline metal materials, and a method formanufacturing the same.

In order to accomplish the above object, an aspect of the presentinvention is to provide a flexible substrate for use in an electronicdevice, wherein the flexible substrate is made of metallic glass havinghigh resilience.

In one embodiment of the present invention, the metallic glasspreferably has a crystallization temperature (T_(x)) of 200° C. orhigher at up to which a process of fabricating an electronic deviceusing the flexible substrate can be conducted, and the metallic glassmay be based on the IIA group element Mg or Ca, the IIIA group elementAl, or the transition metal Ti, Zr, Hf, Fe, Co, Ni or Cu, which are allcommercially available and suitable for use in a continuous process on amass scale.

In another embodiment, the flexible substrate for use in electronicdevices preferably has a thickness of at least 1 μm in order to supportthe electronic devices and up to 500 μm in order to avoid theductile-brittle transition attributable to thickness.

For maximum elastic recovery, the flexible substrate has a yield strainof 1.5% or higher and ranges in strength from 0.3 to 5 GPa and inelastic modulus from 30 to 250 GPa. In a preferred embodiment, theflexible substrate has a resilience of 1.5 MJ/m³ or higher.

In order for the flexible substrate to improve in interfacial propertieswith electronic devices, the flexible substrate ranges in coefficient ofthermal expansion (CTE) from 1 to 20 ppm/° C., which is smaller thanthose of conventional substrates, and has a bending fatigue limit of0.5% or higher.

In accordance with another aspect thereof, the present inventionprovides a method for manufacturing a flexible substrate having highresilience, comprising: preparing materials according to a compositionof a metallic glass; and forming the materials into a metallic glassribbon.

The formation of metallic glass ribbons may include a process ofpreparing metallic glass into a wide ribbon by controlling a meltspinneret nozzle, and a winding process, after which the resultingproduct is applicable to a roll-to-roll process and easy to carry andstore.

The metallic glass ribbons are inosculated with each other or with aheterogeneous material in order to enlarge the area thereof. The areaenlargement may be achieved by inosculating the metallic glass ribbonwith a homogeneous ribbon or with a heterogeneous material through athermo-plastic forming process.

In accordance with a further aspect thereof, the present inventionprovides method for manufacturing a flexible substrate for use in anelectronic device, comprising: preparing materials according to acomposition of a metallic glass; forming the materials into bulkmetallic glass; and processing the bulk metallic glass into a thinplate.

Preferably, the bulk metallic glass can be processed into a thin platesuitable for use as a substrate by thermo-plastic forming process.

The method may further comprise planarizing a surface of the metallicglass substrate. In this regard, the substrate ranges in thickness of500 μm or less in order to avoid the ductile-brittle transitionattributable to thickness.

In accordance with still another aspect thereof, the present inventionprovides an electronic device fabricated with the flexible metallicglass substrate, and the electronic device may be a flexible device.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will be more clearly understood from the following detaileddescription taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a graph comparing the hardness and specific strength of aZr-based metallic glass, manufactured according to one embodiment of thepresent invention, with those of other representative light alloys;

FIG. 2 is a schematic view comparing stress-strain curves of theflexible metallic glass manufactured according to an embodiment of thepresent invention with those of a conventional flexible SUS and polymersubstrate;

FIG. 3 is an image showing the bending behavior of a wide metallic glasssubstrate manufactured according to an embodiment of the presentinvention;

FIG. 4 is a graph showing the strain-fatigue lifetime of the Zr-basedmetallic glass substrate manufactured according to an embodiment of thepresent invention, with inserts schematically representing ranges offatigue strain of the substrate materials upon bending fatigue tests;

FIG. 5 is a graph showing the fatigue endurance limit of the Zr-basedmetallic glass substrate manufactured according to an embodiment of thepresent invention with those of SUS and Polymer substrate;

FIG. 6 is a view showing the correlation between the coefficient ofthermal expansion and tensile modulus for the metallic glass substratemanufactured according to an embodiment of the present invention, aplastic, a typical crystalline metal material, Invar, and quartz;

FIG. 7 is a DSC analysis profile of the Zr-based metallic glasssubstrate manufactured according to an embodiment of the presentinvention;

FIG. 8 is photos showing bending procedure for Zr-based metallic glasssubstrate manufactured according to an embodiment of the presentinvention after annealing at 320° C. during 2 hours; and

FIG. 9 is graphs showing surface morphology and average roughness ofmetallic glass substrate manufactured according to an embodiment of thepresent invention before and after thermos-plastic forming process.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The flexible metallic glass substrate in accordance with the presentinvention is described in detail with reference to the drawings.

The present invention addresses a flexible substrate having highresilience for use in electronic devices, wherein the flexible substrateis made of a commercial amorphous alloy that can be readily produced ina continuous process on a mass scale.

General metals or alloys form a crystalline structure, that is, a highlyordered structure occurring due to the intrinsic nature of itsconstituents to form symmetric patterns, whereas metallic glass(amorphous alloy) is a solid in which constituent atoms are in adisordered arrangement. That is, metallic glass is merely a structure inwhich the positions of constituent atoms in a liquid state are frozen asthey are. Metallic glass is typically prepared by heating a master alloyto reach a liquid state and then quenching the molten alloy at adecrease rate of 10⁵ to 10⁶ K/sec into a solid. In spite of the samecomposition, metallic glass is different in physical properties from acrystalline alloy due to the structural difference therebetween. Inconsideration of the properties required for flexible substrates, thepresent inventor developed metallic glass substrate having highresilience by using commercial metal matrix metallic glass with highresilience as a material of the flexible substrate on the basis of theadvantages and disadvantages of conventional polymer substrates andcrystalline metal substrates.

First, an amorphous alloy is composed of multiple elements, that is, twoor more elements, and typically more than three elements in order toimprove glass-forming ability. In this regard, the amorphous alloy maybe called “Zr-based metallic glass” on the basis of the main elementhaving the highest percentage of the component elements.

Depending on the main constituent and the other constituent elements,the metallic glass varies in characteristic temperatures, such as theglass transition temperature (T_(q)), crystallization temperature(T_(x)), melting point (T_(m)), etc.

For metallic glass, the crystallization temperature is the maximumtemperature at which the alloy still retains its amorphous propertieswhile various processes can be performed. Because the crystallizationtemperature of metallic glass is higher than the maximum temperatureavailable for flexible polymer substrates, metallic glass allows for therelatively easy fabrication of electronic devices without requiringimprovements to existing fabrication processes or the development of newprocesses. Metallic glass that is available as a flexible substratematerial is a commercial metal matrix amorphous alloy system having highresilience, which is suitable for use in a continuous process on a massscale, and may be based on the IIA group element Mg or Ca, the IIIAgroup element Al, or the transition metal Ti, Zr, Hf, Fe, Co, Ni or Cu.

Metallic glass substrates having high flexibility and resilienceaccording to the present invention are explained as follows.

First, free of defective regions existing in crystalline metal, such asgrain boundaries, metallic glass exhibits a high strength (0.3 GPa˜5GPa), which closely approximates the theoretical strength of materialsfor each alloy composition. As can be seen in FIG. 1, heavy Zr-basedalloys, when amorphized, increase in strength and thus have highspecific strength (strength/density), compared to various representativecrystalline light alloys, such as Mg-, Al-, and Ti-based alloys.Materials that have high specific strength, such as these, can satisfythe load value required for a given product even when formed into athinner film. Hence, a material with high specific strength is moresuitable for use in flexible substrates.

In addition, the metallic glass substrate with flexibility in accordancewith the present invention, as shown in FIG. 2, has a yield strain of1.5% or higher, which is far superior to those of conventionalcrystalline SUS metal substrates (0.5% or less), so that the metallicglass substrate can be deformed at a level similar to polymersubstrates. Also, the elastic modulus of the flexible metallic glasssubstrate in accordance with the present invention ranges from 30 to 250GPa, and is lower than those of conventional crystalline SUS metallicsubstrates, which are approximately 250 GPa. Resilience is the abilityof a material to absorb energy when it is deformed elastically, andrelease that energy upon unloading. On the basis of the mechanicalproperties, materials can be compared with regard to resilience in termsof the modulus of resilience (U=ρ_(y) ²/2E). It can be calculated byintegrating the stress-strain curve from zero to the elastic limit. Asis understood from the difference in areas under the stress-straincurves of FIG. 2, the metallic glass of the present invention has a fargreater resilience than do conventional SUS or polymer materials, andcan recover more quickly after elastic deformation.

FIG. 3 is a view illustrating the morphological change and surface shapeof the metallic glass substrate of the present invention when it isartificially transformed. As described above, the metallic glasssubstrate is resilient enough to prevent the occurrence of surfacedefects thereon even at large deformation, and can maintain the originalsplendid appearance.

In order to examine the reliability of substrate in real serviceenvironment, the zirconium-based metallic glass substrate of the presentinvention was subjected to a bending fatigue test, as depicted in FIG.4. All of the invented metallic glass substrates were found to have afatigue limit of 0.5% or higher with regard to strain and to recoverfrom the deformation even when they were repeatedly deformed under astrain greater than the elastic limit of SUS substrates, which meansthat they exhibit higher fatigue endurance limit than those of SUS andPolymer substrate as shown in FIG. 5. Accordingly, the zirconium-basedmetallic glass substrate of the present invention has excellentsustainability thanks to its high resilience and fatigue resistance.These material properties are summarized with conventional substratematerials, SUS 304 and 100HN Kapton, in Table 1, below.

TABLE 1 Zr₅₀Cu₄₀Al₁₀ Zr₆₀Cu₃₀Al₁₀ SUS 100HN Ribbon Bulk Ribbon Bulk 304Kapton^(th) Yield strength (MPa) 1415 1860 1146 1720 290 69 Yield strain(%) 1.73 2.1 1.55 2.2 0.2 3 Young's modulus (GPa) 85 88 74 80 193 2.5Fatigue-endurance limit (MPa) 712 752 486 — 240 40-60 Resilience (MJ/m³)11.8 19.7 8.9 18.5 0.22 0.95

As shown in FIG. 6, the flexible metallic glass substrate of the presentinvention ranges in coefficient of thermal expansion (CTE) from 1 to 20ppm/° C., which is small compared to those of conventional SUS orpolymer substrates. Hence, the flexible metallic glass substrate of thepresent invention can stably endure electronic device fabricationprocesses and can increase production yield and sustainability underuser environments, compared to conventional SUS or polymer substrates.

Moreover, the flexible metallic glass substrate of the present inventionhas a process allowable temperature of 200° C. or higher. As usedherein, the term “process allowable temperature” refers to the maximumtemperature allowable for a process of fabricating an electronic deviceusing a flexible substrate. Having a process allowable temperature of200° C. or higher, the flexible substrate of the present invention canbe used in processes without requiring any additional improvement in theprocesses. Because the upper limit of the process allowable temperatureof metallic glass, as shown in the differential thermal analysis curveof FIG. 7 for the Zr-based metallic glass substrate of the presentinvention, is determined by the crystallization temperature (T_(x)), atwhich the crystallization reaction initiates the release of energy uponheating, the upper limit varies depending on the material composition.Accordingly, when a high-temperature process is employed during thefabrication of an electronic device, a metallic glass material with ahigh crystallization temperature may be used as a substrate.

FIG. 8 shows bending procedure for Zr-based metallic glass substratemanufactured according to an embodiment of the present invention afterannealing at 320° C. during 2 hours, which is higher than processallowable temperature of conventional polymer substrate and require tobe used in electronic device manufacturing processes on the substratewithout requiring any additional improvement in the processingprocedure. As shown in FIG. 8 (a)-(d), the ribbon exhibit perfectelastic behavior even after high temperature annealing, which means thatthe metallic glass substrate of the present invention exhibit higher“process allowable temperature”.

As described above, the flexible metallic glass substrate having highresilience in accordance with the present invention is free of thedisadvantages of conventional materials, but is provided with theadvantages conferred by conventional materials. These materialproperties are summarized in Table 2, below.

TABLE 2 Plastic SUS Metallic Glass Strength <0.1 Gpa ~0.3 Gpa 0.3-5 GpaThickness Tens of μm- tens of μm- ones of μm- ones of mm hundreds of μmhundreds of μm Yield 2%< <0.5% 1.5%< strain Elastic 2~4 GPa ~250 GPa30~250 GPa Modulus CTE ~25 ppm/° C. ~20 ppm/° C. 1~20 ppm/° C. Process100~300° C. ~1000° C. 200~800° C. Temp.

The method for manufacturing the metallic glass substrate of the presentinvention will be described below.

As long as it is typically used to manufacture metallic glass, anymethod may be applied without limitation. For example, a method may beselected from among melt-spinning, injection casting, thermal plasticforming, water quenching, high-pressure die casting, copper moldcasting, cap-casting, suction-casting, squeeze-casting, arc-melting,zone melting, (single or twin) roll casting, and mechanical alloying.

Metallic glass is broadly divided into metallic glass ribbon and bulkmetallic glass, depending on how it is formed. When formed into aribbon, the metallic glass is thin. On the other hand, bulk metallicglass needs to be processed to form a thin plate. For use in electronicdevices, the flexible metallic glass substrate preferably has athickness of at least 1 μm in order to support the electronic devicesand up to 500 μm in order to avoid the ductile-brittle transitionattributable to thickness.

The formation of metallic glass ribbons may include a process ofpreparing metallic glass into a wide ribbon by controlling a meltspinneret nozzle, for example using planar flow casting method, and acoil winding process in order to be applicable to a roll-to-roll processand easy to carry and store. When narrow, the metallic glass ribbons arelaterally inosculated with each other or with a heterogeneous materialin order to enlarge the area thereof.

No particular limitations are imposed on the formation of bulk metallicglass into thin plates or the area enlargement of metallic glass ribbon.So long as it is used in the art, any method may be applied. Arepresentative process is a thermo-plastic forming process using asupercooled liquid region, a characteristic temperature range ofmetallic glass.

The thermo-plastic forming process is performed by preheating a materialin a short period in a supercooled liquid temperature region from glasstransition temperature to crystallization temperature and pressurizingthe material, while maintaining amorphous state of the material. Withthis process, bulk metallic glass can be formed into a thin plate.

In addition, metallic glass ribbons may be partially overlapped witheach other or with a heterogeneous material and pressurized to give anenlarged area in a supercooled liquid temperature region.

The flexible metallic glass substrate prepared in accordance with thepresent invention has a smoother surface than do conventional flexiblemetallic substrates, but for an even smoother surface, a planarizationprocess may be performed on the flexible metallic glass substrate. FIG.9 shows surface morphology and average roughness profile of metallicglass substrate manufactured according to an embodiment of the presentinvention before and after thermos-plastic forming process. The measuredsurface roughness of the metallic glass substrate by atomic forcemicroscope changes from about 203 nm to 3.27 nm before and afterthermo-plastic forming process. As described above, the metallic glasssubstrate can be smoother and planarization after thermos-plasticforming process.

In accordance with another aspect thereof, the present inventionaddresses an electronic device fabricated using the flexible metallicglass substrate of the present invention.

The electronic device is not particularly limited. The flexible metallicglass substrate of the present invention may be applicable to allpossible electronic devices, as exemplified by lighting devices, displaydevices, thin film transistors, microprocessors, and solar cells.Particularly, the flexible metallic glass substrate is suitable for usein a flexible electronic device that is itself bendable.

Made of commercial amorphous metal alloys, as described above, theflexible metallic glass substrate for electronic devices exhibits highresilience and is provided with advantages conferred by polymermaterials and metal materials, but is free of the disadvantages ofconventional materials.

Although the preferred embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

What is claimed is:
 1. A flexible substrate for use in an electronicdevice, wherein the flexible substrate is made of metallic glass havinghigh resilience.
 2. The flexible substrate of claim 1, wherein themetallic glass is a material selected from among Mg-, Ca-, Al-, Ti-,Zr-, Hf-, Fe-, Co-, Ni-, and Cu-based metallic glass.
 3. The flexiblesubstrate of claim 1, ranging in strength from 0.3 to 5 GPa and inelastic modulus from 30 to 250 GPa, and having a yield strain of 1.5 orhigher.
 4. The flexible substrate of claim 1, having a resilience of 1.5MJ/m^(J) or higher.
 5. The flexible substrate of claim 1, wherein themetallic glass has a crystallization temperature of 200° C. or higher.6. The flexible substrate of claim 1, ranging in thickness from 1 to 500μm.
 7. The flexible substrate of claim 1, ranging in coefficient ofthermal expansion (CTE) from 1 to 20 ppm/° C.
 8. The flexible substrateof claim 1, having a bending fatigue limit of 0.5% or higher.
 9. Amethod for manufacturing a flexible substrate having high resilience,comprising: preparing materials according to a composition of a metallicglass with high resilience; and forming the materials into a metallicglass ribbon.
 10. The method of claim 9, wherein the forming step iscarried out by controlling a melt spinneret nozzle to form a metallicglass ribbon having a wide width.
 11. The method of claim 9, furthercomprising winding the metallic glass ribbon.
 12. The method of claim 9,further comprising inosculating the metallic glass ribbon with anotherto achieve area enlargement.
 13. The method of claim 12, wherein thearea enlargement is achieved by inosculating the metallic glass ribbonwith a homogeneous ribbon or with a heterogeneous material through athermo-plastic forming process.
 14. A method for manufacturing aflexible substrate for use in an electronic device, comprising:preparing materials according to a composition of a metallic glass withhigh resilience; forming the materials into bulk metallic glass; andprocessing the bulk metallic glass into a thin plate.
 15. The method ofclaim 14, wherein the forming step is carried out in a thermo-plasticforming process.
 16. The method of claim 9, further comprisingplanarizing a surface of the substrate.
 17. The method of claim 9,wherein the substrate ranges in thickness from 1 to 500 μm.
 18. Themethod of claim 14, further comprising planarizing a surface of thesubstrate.
 19. The method of claim 14, wherein the substrate ranges inthickness from 1 to 500 μm.
 20. An electronic device, fabricated withthe flexible substrate of claim 1.